Load Combination for Overturning due to Live Load

[mqr7]

Hello everyone, I have an equipment pad with embedded posts that are subject to impact from vehicles. I have always multiplied the "vehicle barrier load" per ASCE 7 by the 1.6 live load factor, but the equipment pad is also now subject to overturning and sliding under this load. I am unsure of what load combination would be most appropriate to use under this loading scenario. For reference, the pads are ~20'x~5'x12" so they are quite large. I do not believe the 1.2D+1.6L load combo is appropriate since that amplifies the dead (resisting) load. Currently my thought is to compare the 1.6L to 0.9D but that seems overly conservative to me since I know the equipment and the weight of the pad will be there during impact.

ETA: I am also not sure whether the D+L combo allowable combo is appropriate since I don't see "impact" as an "allowable" level load due to the uncertainty of it, but overturning and sliding are not strength level checks. Appreciate any thoughts. Thanks.

 

[zurch1818]

It's not exactly your situation but I work a lot in telecommunications where carriers put equipment of known weight on rooftop platforms. Although this equipment weight is known, it is technically live load. For the purposes of calculations, I call it dead load as it seems overly conservative when the uncertainty of the weight isn't there.

The other thing to consider is the reduced dead load cases have the FOS built into them. When it comes to overturning, you need to have a FOS. The ASCE equations essentially have a 1.67 FOS built into them (both ASD and LRFD).

There is also the old school ASD 1.5 FOS if you want to rationalize it differently. In my industry, this FOS is still predominantly used for ballast frames on RTs. Maybe ASD could help you out (even if you don't use the 1.5 FOS)? I don't ever deal with impacts, so I don't know what the corresponding ASD factor is off the top of my head.

 

[mqr7]

@zurch1818 I agree with you regarding the FOS. I suppose I could consider the 0.9D+1.6L, akin to the old school 0.9D+1.6W LC and check it to unity and that would satisfy the FOS.

 

[TRAK.Structural]

@zurch1818 I agree with you regarding the FOS. I suppose I could consider the 0.9D+1.6L, akin to the old school 0.9D+1.6W LC and check it to unity and that would satisfy the FOS.

Yes